Well the basic B-29 design was developed into the Stratoliner, the world first pressurised airliner, the C-97 cargo plane, and the KC-97, the first production airborne tanker.
The B-17 was never developed beyond the basic bomber variant. Some were converted into target drones and what not, and I think a few were modifed shortly after the war as water bombers, but the design never really lent itself to public commercial applications such as an airliner, not when the DC-3, DC-4, DC-6, Lockheed Constellation, and the Stratoliner were either in service or coming into service at the end of the war and the years immediately afterwards.
Also the OXCART isn't the name of the A-12, that's the name of the CIA program that funded and flew the plane. The A-12 was never given an "official" name.